From: Robert Klemme <bob.news@...>
Date: 2004-03-29T20:54:23+09:00
Subject: Re: YAML and initializers


"why the lucky stiff" <ruby-talk@whytheluckystiff.net> schrieb im
Newsbeitrag news:4067F665.1030500@whytheluckystiff.net...
> Good discussion, Hal.  I've been really rethinking the YAML+Ruby typing
> mechanism, so this is a perfect time for this.  Right on.
>
> Hal Fulton wrote:
> >
> > When entities have default values in the constructor, we can omit them
> > when calling #new.
> >
> > I'd like to be able to omit these in YAML also. (I can omit them now,
> > of course, but they get nil values).
> >
>
> So, if I understand you correctly, you use default values in the
> constructor to enforce default values for object properties.  In the
> case below:
>
> >
> >   require "yaml"
> >
> >   class MyClass
> >     def initialize(a,b,c=5)
> >       @alpha, @beta, @gamma = a,b,c
> >     end
> >   end
> >
>
> ..the use of c=5 is a way of ensuring that 5 is the default value for
> @gamma.
>
> >
> > A part of me wishes that YAML could somehow be smart enough to call
> > the constructor, maybe with a special call like 'construct' instead
> > of 'load' -- but there are likely ramifications I haven't thought of.
> >
>
> In the case of the above constructor, it would be impossible for YAML to
> know in what order to pass the three parameters and I don't have any
> ideas for making the YAML loader work with the constructor.
>
> I think the best solution for this would be akin to
> Object#initialize_copy.  In fact, it seems like YAML could call
> Object#initialize_copy with the new object, should an
> Object#initialize_copy method exist.  Yeh?
>
> Yes, you can also do this with a typing callback:
>
>    YAML::add_domain_type( "hypermetrics.com,2004", "MyClass" ) do
>        |type, val|
>        val['gamma'] ||= 5
>        YAML::object_maker( MyClass, val )
>    end
>
>    YAML::load( <<EOY )
>    --- !hypermetrics.com,2004/MyClass
>    alpha: 10
>    beta: 20
>    EOY
>    #=> #<MyClass:0x82418c0 @alpha=10, @gamma=5, @beta=30>

As food for thought: This could be extended into something more general:

class MyClass
  DEFAULTS = {:gamma => 5}

  def initialize(a,b,c=DEFAULTS[:gamma])
    @alpha, @beta, @gamma = a,b,c
  end
end

YAML::add_domain_type( "hypermetrics.com,2004", "MyClass" ) do
  |type, val|

  scheme, domain, name = type.split(/:/)
  t = Object.const_get name

  if t.const_defined? :DEFAULTS
    t::DEFAULTS.each do |sym, v|
      val[sym.to_s] ||= v
    end
  end

  YAML::object_maker( t, val )
end

YAML::load( <<EOY )
--- !hypermetrics.com,2004/MyClass
alpha: 10
beta: 20
EOY

    robert